It is proposed to study in depth, the muscle satellite cells, with emphasis on the following objectives: 1. The morphology and distribution of satellite cells in neonatal muscle. 2. Determination of the effect of pathologic processes and trauma (dystrophy and denervation) on the quantity and ultrastructure of muscle satellite cells. 3. Comparison of satellite cell dynamics in myogenesis and growth of dystrophic and normal muscle. 4. Investigation of the origin and fate of satellite cells in degenerating and regenerating muscle. 5. Analysis of the significance of the satellite cell in muscular dystrophy. In vivo studies of two genetic mutant mouse strains (C57BL/6J-dy2J and 129/Re-dy) will be carried out using light and electron microscopy and light microscopic radioautography.